Solvatochromism inversion

A sampling of appHcations of Kamlet-Taft LSERs include the following. (/) The Solvatochromic Parameters for Activity Coefficient Estimation (SPACE) method for infinite dilution activity coefficients where improved predictions over UNIEAC for a database of 1879 critically evaluated experimental data points has been claimed (263). (2) Observation of inverse linear relationship between log 1-octanol—water partition coefficient and Hquid... 

A thermochromic shift is the displacement of an absorption or emission band with the temperature of the solvent. These displacements result from the change in solvent polarity with temperature, the general rule being that the polarity decreases as the temperature increases. These shifts are small compared with solvatochromic effects and are unlikely to lead to state inversion (Figure 3.52). 

Scheme 13 Molecules for which inversion of solvatochromism has been claimed.

The results show that in polar solvents, the 2max of POT is indeed blue-shifted with respect to the PANI peak. PANI has consistently been found to have a higher solvatochromic coefficient than POT. This is to be anticipated, since the solvation energy is inversely proportional to the volume of the solute (AE a 1/r ) and thus the presence of the methyl group in POT,... 

The three scales devised by Kamlet, Abboud and Taft have been used many times to formulate relationships between reaction rate constants and solvent polarity. These are known as linear solvation energy relationships (LSERs). The rate of amide formation for example, the most common single reaction in medicinal chemistry, is inversely proportional to jS for entropic reasons (Figure 3.4). Limonene and its derivative p-cymene were thus justified as excellent options for a renewable amidation solvent, not only in terms of performance but also because they are produced from a renewable feedstock. Other solvents are less suitable according to their solvatochromic polarity parameters (Table 3.3). As hydrocarbons, some aquatic toxicity concerns surround the use of limonene and p-cymene, but ideally these would be minimised with recycling. 

Figure 10.3.4. Examples of the solvatochromic molecules a) positive solvatochromic paranitroaniline, b) negative Reichardt s dye and c) inversely solvatochromic Brooker s merocyjanine.

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